An x-ray tomography system can provide information of the internal structure of a sample without having to destroy or cross-section the sample. The X-rays are passed through the sample and detected by an x-ray detector to obtain an absorption image from a cross-section of the sample. The x-ray detector acquires a two-dimensional x-ray shadow (absorption-contrast) image. For three-dimensional tomography, the sample and/or x-ray source and detector may be incrementally rotated to enable cross sectional images to be obtained from multiple angles. The set of cross-sectional images obtained in this manner can then be mathematically manipulated to reconstruct a 3D image of the sample.
The term: “SEM-based nanotomography system” comprises any system using an electron beam impinging on a target to generate x-rays and includes a range of x-ray systems designed for very high resolution 2D x-ray imaging and 3D x-ray tomography. Such systems differ from the more common x-ray systems used for lower-resolutions (e.g., >1 μm) in their use of an electron beam produced by a scanning electron microscope (SEM), transmission electron microscope (TEM), or scanning transmission electron microscope (STEM), to generate x-rays from a virtual x-ray source typically smaller than ˜500 nm. As the electrons in the beam impact the target, x-rays are generated.
Two important parameters of an x-ray tomography system are resolution and throughput. Throughput refers to how fast the system can acquire an image or set of images. While there are many definitions of “resolution,” it typically refers to how close two objects can be and still be distinguished as two distinct objects. Resolution affects how small a feature the system can image. Throughput can be increased by increasing the flux of x-rays passed through the sample, which may be achieved by increasing the electron beam current impacting the target. However, the impact of the electron beam with the target can cause excessive heating and even melting of the target, thus electron beam currents may be limited to less than 1-2 μA for beam energies up to 60 keV. Because the electron beam diameters are in the sub-micron range, extremely high energy densities typically arise within the targets.
While some x-ray generating systems use focusing optics to focus the generated x-rays, other systems do not use focusing optics. The resolution of an x-ray tomography system without x-ray focusing optics is determined in large part by the effective (“virtual”) size of its x-ray source. For systems using an electron beam to generate x-rays, the effective source size is often determined by the volume within which the beam electrons interact with and come to rest in the target. This interaction volume is largely determined by the density and atomic number of the target material, and the diameter and energy of the electron beam, and is typically tear-drop shaped. The design of the x-ray target is a critical determinant of the system performance with respect to resolution, imaging time, and image signal-to-noise ratios, as well as other operational considerations.
The original targets employed in these systems were “bulk targets”—typically pieces of target material (usually metals such as tungsten, molybdenum, titanium, scandium, vanadium, silver, or other a refractory metal, etc.) which were much larger than the diameter of the electron beam used to induce x-ray emission. Since the great majority of the energy of the electron beam striking the target goes into producing heat (not x-rays), the high cooling efficiency of these types of targets was advantageous in maximizing x-ray fluxes and thus throughputs. However, imaging resolutions were limited by the spreading of the electron beam within the target material, since the electron dissipation volume also corresponds to the virtual source size, and thus resolutions were limited.
More recently, a new type of x-ray target has been employed in SEM-based nanotomography systems, as disclosed in U.S. Pat. Publ. No. 2015/0303021, “High Aspect Ratio X-Ray Targets and Uses of Same”, which is assigned to the assignee of this invention. Post targets comprise very small, approximately rectangular (cross-section), targets having dimensions typically <500 nm in the two axes which determine the virtual source size (e.g., Y-Z in FIG. 3D) and are about 1-2 μm long (X-dimension), attached to a support base. The diameter of the beam spot at the target may be larger than the largest dimension (e.g., diameter of a cylindrical post or diagonal of a rectangular prism post) of a cross section of the post. The support base may be composed of a material such as silicon, aluminum, beryllium, or other low atomic number material, which generates fewer x-rays upon impact of the electron beam than the target material. An electron beam striking the end of a post target will generate x-rays with a virtual source size controlled by the Y-Z dimensions of the post, rather than the electron dispersion distance as in a bulk target—thus higher imaging resolutions are possible with post targets than with bulk targets. However, due to the small cross-sectional area of the posts, thermal conductivity is much lower than for a bulk target, and thus excessive heating presents a limit to the allowable electron beam energy which may be deposited into the target before melting may occur (even for tungsten targets).
Hence, both the commonly-used bulk targets, and the more recently-developed post targets present difficulties for optimizing the resolution and throughput of a SEM-based nanotomography system. Embodiments described in FIGS. 4A-5B address some of these concerns, with FIG. 9 providing a comprehensive summary of the source parameters of the various source types.
FIGS. 1A-1D show four views of the operation of an x-ray source employing a bulk target 108 impacted by a round electron beam 106. Bulk targets typically comprise a target material, such as tungsten, molybdenum, titanium, scandium, vanadium, silver, or other a refractory metal, etc., which is relatively large in X-Y-Z dimensions compared to the anticipated size of the electron beam which will be directed towards the target to generate x-rays.
FIG. 1A is an isometric view of a bulk target 108 with an upper surface 104 being bombarded by electrons 102 in an electron beam which has been focused into a round electron beam 106 by an electron column (not shown) such as would be found in a scanning electron microscope (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), or any electron beam system capable of focusing a high current density electron beam onto a target. As a result of the impact of electrons 102 with bulk target 108, x-rays are generated which propagate out in all directions (into a 4π steradians solid angle). X-ray flux 112 represents only the very small fraction (typically 0.1 to 0.4%) of the total x-ray flux which is emitted through the front surface 110 of target 108 in a direction towards an x-ray detector (see FIG. 2). The great majority of the total x-ray flux (not including x-rays 112) generated by impact of electron beam 106 with the bulk target 108 will be emitted in directions away from the x-ray detector and thus will not contribute to x-ray imaging.
FIG. 1B is a top view (in the X-Y plane) of the x-ray source from FIG. 1A, illustrating two cross-sections A-A and B-B, which are presented in FIGS. 1C and 1D, respectively. Also illustrated is the dissipation of heat 122 generated in target 108 by absorption of kinetic energy from electrons 102. Typically, almost all the electron energy produces heat, with only a small fraction of the electron energy producing x-rays. Because the electron beam 106 is relatively small compared to the dimensions of the bulk target, heat 122 may dissipate out rapidly by thermal conduction over an ˜180° range (i.e., not through surface 110) in the X-Y plane within target 108 as shown.
FIG. 1C is a side cross-sectional view A-A (in the X-Z plane) of the x-ray source from FIGS. 1A-1B. The penetration distance of electrons 102 into the target 108 is typically a bulb-shaped volume 142 with a depth down from top surface 104 determined by the energy of electrons 102. For high energy electrons 102 the penetration depth may be microns, while for 1.5 keV electrons the penetration depth may be only ˜50 nm. A fraction of the x-rays generated within target 108 may be reabsorbed within target 108 before reaching exit surface 110. In the case of ˜0.5 keV x-rays generated in a titanium target fewer than 20% will typically be absorbed within target 108. As can be seen, heat 122 may dissipate out over an ˜90° range (i.e., not through surfaces 104 or 110) in the X-Z plane from the electron penetration volume 142.
FIG. 1D is a front cross-sectional view B-B (in the Y-Z plane) of the x-ray source from FIGS. 1A-1B. The penetration volume 162 in general will look much like penetration volume 142 in FIG. 1C for cases where the electron beam 102 impacting target 108 is round. As can be seen, heat 122 may dissipate out over an ˜180° angular range in the Y-Z plane from the electron penetration volume 162.
The wide angular ranges of heat dissipation 122 illustrated in FIGS. 1B-1D show that heat dissipation within a bulk target can be rapid going away from the electron beam (impact region) 106 of the electrons 102 with target 108. Thus, relatively high electron beam powers may be employed, enabling larger x-ray fluxes to be generated. A typical “rule of thumb” for x-ray generation is that x-rays produced by an electron beam will have energies ranging up to about ⅓ of the energy of the electron beam, thus a 30 keV electron beam will efficiently generate x-rays mostly below ˜10 keV, while a higher energy 60 keV electron beam will generate x-rays mostly with energies below ˜20 keV. For some applications, however, much lower x-ray energies may be preferred and will be discussed below.
A bulk target is thus able to dissipate heat generated in the target from the impact of the electron beam. A disadvantage of a bulk target is that the virtual source size is determined by the penetration volume (142 in FIG. 1C and 162 in FIG. 1D). The achievable imaging resolution is fundamentally determined by the source size, thus the larger the source size, the lower the resolution in 2D x-ray images or in 3D x-ray tomographic reconstructions. These issues are discussed further in a subsequent section. Table 900 in FIG. 9 summarizes many of the source parameters just discussed. Typically, the bulk target may be attached to a support structure, which, in turn, may be affixed to a support arm, as illustrated in FIG. 8.
FIG. 2 shows a top view (in the X-Y plane) projection x-ray imaging arrangement 200. X-rays are emitted from a target 202 due to the impact of an electron beam (not shown), as illustrated in FIGS. 1A-1D. X-rays generated in target 202 are emitted in all directions (4π steradians), however a small fraction (typically 0.1 to 0.4%) 204 of the total x-ray flux is emitted into the solid angle subtended by the x-ray detector 208 with respect to target 202. For x-ray imaging or x-ray tomography of a sample 206, a portion of x-rays 204 will pass through sample 206, with a fraction of the x-rays being absorbed by sample 206, thereby producing a shadow image at detector 208. The flow chart in FIG. 10 describes a method for x-ray imaging or x-ray tomography employing an experimental configuration as shown in FIG. 2.
FIGS. 3A-3D show four views of an x-ray source employing a post target 308 impacted by a round electron beam 306. Post targets typically comprise some of the same target materials as bulk targets, such as tungsten, molybdenum, titanium, scandium, vanadium, silver, or a refractory metal, etc. However, unlike the situation for bulk targets, post targets comprise a very small structure elongated along the desired x-ray emission direction (i.e., towards the sample and detector —see FIG. 2). The virtual source size for a post target is determined by the transverse dimensions (i.e., dimensions along the Y- and Z-axes perpendicular to the X-axis of x-ray emission towards the sample and detector—see FIG. 2). The transverse dimension in the Y direction may be smaller than the diameter of the electron beam, so that the size of the virtual x-ray source in the Y direction is determined by the Y dimension of the target and not by the diameter of the electron beam.
Post target 308 is supported on a support structure (not shown) which would be at the upper left of FIG. 3A. To avoid x-ray generation 312 from the support structure due to impact by electrons 302 in electron beam 306, at least two approaches are possible. In some embodiments, the support structure may be fabricated from a material that is less efficient (typically lower atomic number, such as silicon, aluminum or beryllium) at x-ray generation than the material of post target 308, thus even if the edge of electron beam 306 impacts the support structure, minimal extraneous x-ray generation will result which might affect the resolution in the x-ray image. In other embodiments, the support structure may be fabricated from the same material as post target 308, however, post target 308 would be fabricated with sufficient length (X-axis) to ensure that the electron beam 306 only impacts post target 308 and not the support structure. These two approaches may be combined in still other embodiments.
FIG. 3A is an isometric view of a post target 308 with an upper surface 304 being bombarded by electrons 302 in an electron beam which has been focused into a round electron beam 306 by an electron column such as would be found in a scanning electron microscope (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), or any electron beam system capable of focusing a high current density electron beam onto a target. Unlike the case for electron beam 106 in FIGS. 1A-1D, here some electrons 302 may miss the narrow post target 308, travelling past target 308 on either side and thus not producing x-rays. In addition, because the Z-axis thickness of the post target may be smaller than the penetration depth (such as penetration depths 142 or 162 in FIGS. 1C and 1D, respectively), some of electrons 302 may travel out the lower surface of post target 308. Because some electrons 302 may not impact the post target 308 and some other electrons 302 may pass through the post target 308, x-ray generation by post targets may be less efficient (measured in x-ray flux for a given electron beam current) than for a bulk target. As was the case for the bulk target 108 in FIGS. 1A-1D, the impact of electrons 302 with post target 308 generates x-rays which propagate out in all directions (into a 4π steradians solid angle). X-ray flux 312 represents the very small fraction (typically 0.1 to 0.4%) of the total x-ray flux which is emitted through the front surface 310 of target 308 in a direction towards an x-ray detector (see FIG. 2).
FIG. 3B is a top view (in the X-Y plane) of the x-ray source from FIG. 3A, illustrating two cross-sections C-C and D-D, which are presented in FIGS. 3C and 3D, respectively. Also illustrated is the dissipation of heat 322 generated in target 308 by absorption of kinetic energy from electrons 302. Typically, almost all the electron energy produces heat, with only a small fraction of the electron energy producing x-rays, however for a given electron beam current, less heat will be deposited into the post target 308 than would typically be the case for a bulk target 108 due to the two factors discussed above: electrons missing the target, and electrons passing clear through the target. As is shown schematically in FIGS. 3B-3C, heat dissipated in the post target 308 is not transmitted efficiently away from the impact region of electron beam 306, as illustrated by dissipation arrows 322 where heat may only be conducted down the length of the post target 308 (in the x-direction only), not away in both the X- and Y-directions as was the case for the bulk target 108. Thus, although a post target may absorb somewhat less heat from the electron beam, the temperature rise of the end of post target 308 (at electron beam 306) may be much larger than for a bulk target 108 impacted by the same electron beam current at the same beam voltage.
FIG. 3C is a side view cross-section C-C (in the X-Z plane) of the post target x-ray source from FIGS. 3A-3B. As discussed above, since the Z-axis dimension of the post target 308 is smaller than a typical penetration distance of electrons 302, electrons 302 may pass entirely through post target 308, emerging out the bottom as shown. Thus, unlike the situation for a bulk target 108, for energetic electron beams (e.g., with energies above ˜5 keV), x-rays may be emitted from the full Z-axis dimension of post target 308 (region 342), and the Z-axis source size would then correspond to the Z-axis dimension of post target 308. For lower energy electron beams (e.g., 1.5 keV), where the penetration depth may be only ˜50 nm, the electron beam may stop within a post target 308 having a Z-axis dimension exceeding the penetration depth (i.e., a Z-axis dimension >50 nm for 1.5 keV electrons). A fraction of the x-rays generated within target 308 may be reabsorbed within target 308 before reaching exit surface 310.
FIG. 3D is a front view cross-section D-D (in the Y-Z plane) of the post target x-ray source from FIGS. 3A-3C. The virtual source size will essentially correspond to the surface 310.
The narrow angular ranges of heat dissipation 322 illustrated in FIGS. 3B-3D show that heat dissipation within a post target may be substantially reduced compared with the bulk target in FIGS. 1A-1D. Thus, relatively lower electron beam powers (decreased electron beam currents, although possibly at the same beam voltage) may be required relative to bulk targets to avoid target melting, thereby generating lower x-ray fluxes. Since for a post target the virtual source size is determined by the Y- and Z-axis dimensions of the post, which may be substantially smaller than the penetration depths 142 and 162 in a bulk target 108 (which determine the source size for the bulk target), higher 2D imaging and 3D tomographic resolutions may be achievable with a post target, although possibly with longer image acquisition times arising from the lower x-ray fluxes produced by post targets. Typically, the post target may be attached to a support structure, which, in turn, may be affixed to a support arm, as illustrated in FIG. 8. The X-Y-Z dimensions of post target 308 may be determined by the following criteria:
X-dimension (length perpendicular to the electron beam)—preferably large enough to prevent the electron beam from impacting the support structure, but also preferably not so long as to make thermal conduction poor from the end (where the electron beam 306 impacts).
Y-dimension (width perpendicular to the electron beam)—comparable to the desired Y-axis resolution in the x-ray image.
Z-dimension (depth parallel to the electron beam)—comparable to the desired Z-axis resolution in the x-ray image.
Thus, it would be advantageous to improve throughput while maintaining resolution in an x-ray imaging system.
It would also be advantageous to provide an improved x-ray target design with improved throughput in the generation of low-energy x-rays while maintaining a sub-micron virtual source size.
It would in addition be advantageous to provide an x-ray imaging system with improved imaging performance employing low energy x-rays to enable x-ray imaging in the energy range corresponding to the “water window”, which is a region of the x-ray spectrum between 280 and 530 eV (x-ray energy) where natural contrast occurs between water and biological materials.